So as they planned their response to the Aug. 21 chemical-weapons attack, Obama's aides focused on calibrating a response that would send a message without causing wider chaos. Too much force could alter the strategic balance of the conflict, possibly empowering Islamist rebels--some of whom are allied with al-Qaeda--whom the White House considers more dangerous than Assad himself. Use too little force, however, and you look feckless. Warns Scowcroft: "Nothing would be worse than to make a gesture which changed nothing and made us look even more impotent."

A Message to Tehran

On Dec. 10, 2009, Obama flew to Norway to accept the Nobel Peace Prize, a premature honor that was not entirely welcomed by the White House. By then Obama had already sent more troops to Afghanistan, escalated drone strikes against al-Qaeda terrorists and seen his hand of friendship rejected by Tehran. It was growing clear that the world might not transform after all. So even as Obama celebrated the cause of world peace that day, he acknowledged that a dangerous world sometimes calls for war, usually led by American arms. "Whatever mistakes we have made, the plain fact is this: the United States of America has helped underwrite global security for more than six decades with the blood of our citizens and the strength of our arms," he said.

Obama is applying that principle now in Syria. Whatever comes of Obama's confrontation with Assad, an even more dangerous confrontation lies in wait--the one with Iran. If another round of negotiations with Tehran should fail, Obama may soon be obliged to make good on his vow to stop Iran from developing a nuclear weapon. "I will not hesitate to use force when it is necessary to defend the United States and its interests," Obama told the American Israel Public Affairs Committee in March 2012.

But to his critics, Obama does hesitate, and trouble follows as a result. With more than three years left in his presidency, he has the opportunity to reverse that impression. Success in Syria and then Iran could vindicate him, and failure could be crushing. "The risk is that, if things in the Middle East continue to spiral, that will become his legacy," says Brian Katulis, a former Obama campaign adviser now with the Center for American Progress.

Some Democratic Presidents have been crippled by foreign policy: Carter by Iran, Lyndon Johnson by Vietnam. But there is another model. Clinton doused the fires in the Balkans and demonstrated the nobility of American intervention. Obama has time to find a path through the current chaos to a successful legacy abroad.

As he charts his course, he might consider a thought from an unlikely source. In a 2009 British newspaper interview that struck a moderate tone, Assad said he hoped Obama would take an active role in the Middle East peace process because only Washington could broker a lasting solution. He said, "There is no substitute for the United States."